Tuned circuits in radios have one severe limitation - bandwidth. Without going into a complex explanation let us assume that the best response can be about 2% of the signal frequency. In the early days of a.m. radio, circuits simply tuned straight across the frequency band of interest.
Applying our 2% rule we find at say 540 Khz, the bandwidth is 10.8 Khz. We would be able to receive this signal without a great deal or little interference from adjacent channels. On the downside if we wanted to receive a signal at say 1550 Khz our bandwidth becomes 31 Khz or spanning 3 channels. We would have little hope of satisfactorily receiving a signal because our bandwidth also now includes both adjacent channels.
A method of receiving called the 'superhetrodyne' principle evolved.
Here as part of our receiver we have a 'local oscillator' or mini transmitter where the incoming received signal is mixed with the local oscillator. As a result 4 frequencies become available.
Firstly the original signal, (2) then the original local oscillator signal, (3) then the original signal plus the local oscillator signal and then finally (4) the original signal minus the local oscillator signal.
Confused?. Consider this practical example of your little transistor a.m. radio. It is designed to receive about 540 - 1650 Khz. The local oscillator will always tune in tandem with the input section to produce another signal at 995 - 2105 Khz.
At all times the difference frequency is a constant 455 Khz or what is called the intermediate frequency or I.F. All other frequencies arising from this process are then filtered out.
When you tune your radio you are actually tuning the local oscillator which is more correctly called the 'V.F.O.' or variable frequency oscillator.
Because we always have a constant difference frequency of 455 Khz it is relatively easy to design and construct narrow band circuits to suit our requirements. It is in these circuits (I.F.